Certain wastes introduced into a septic tank tend to separate into distinct layers: a bottom sludge layer, a top scum layer, and a noticeably distinct intermediate liquid layer that is to a large extent free of scum and sludge. U.S. Pat. No. 5,531,894 describes a septic tank and a system for pumping effluent out of the tank. An electricmotor-operated effluent pump is disposed generally upright and coaxially within an upright main tube that extends downward from a top wall of the tank to a location at or near a bottom wall of the tank. Hence, the tube will pass through the top scum layer, through the intermediate liquid layer, and into the bottom sludge layer. Operation of the pump is controlled by sensors that sense the level of waste in the tank. When the level rises beyond an upper limit, the pump operates to pump fluid out of the tank until the level drops to a lower limit at which the pump shuts off.
Because of the nature of certain electric-motor-operated effluent pumps, it is important that liquid flows along the exterior of the pump/motor when the motor operates the pump so that heat generated by the electric motor is transferred to the liquid. Hence, a known electric-motor-operated pump for a septic tank comprises a tubular sleeve within which the pump and motor are coaxially disposed, and it is this sleeve that is disposed coaxially within the aforementioned upright main tube that extends downward from the top wall of the tank.
For various reasons, it is desirable to maintain the waste level within the tank between the upper and lower limits by pumping liquid from the intermediate zone, rather than drawing from either the sludge or scum layers. Accordingly, it is known to provide openings through the side wall of the main tube at the intermediate layer so that liquid from that layer can enter the main tube and pass downward through the main tube to the bottom of the pump/motor sleeve where it can then be drawn upward through the sleeve by operation of the pump. As the pump operates, liquid is drawn upward across the exterior of the electric motor that operates the pump and enters the pump inlet. The pump pumps the liquid out through an effluent pipe connected to the pump outlet, thereby conveying the effluent out of the tank. The liquid passing along the exterior of the motor serves to provide some cooling of the motor.
Because the intermediate layer may contain materials that are other than pumpable liquid, the intermediate layer liquid is filtered before it reaches the pump. It is known to use stationary screens for such filtering, as shown by the aforementioned and other U.S. Patents such as U.S. Pat. Nos. 5,635,064 and 5,492,635.